1. Field of the Invention
The invention relates to a method for heat-treating profiled rolling stock, in particular track or railroad rails, with an increased heat removal from portions of the profile surface during cooling in the gamma range of the basic iron material, wherein a conversion into a fine pearlitic grain of increased strength, in particular increased wear resistance and increased hardness takes place in the desired cross-sectional area(s), particularly in the head area of rails, and, if required, a deformation or bending by a thermally caused warping of the rolling stock, in particular the rail, perpendicularly to the longitudinal axis is decreased, preferably essentially prevented, during cooling to room temperature, particularly following a structural conversion in the more heavily cooled cross-sectional area(s), and an increased rigidity and fatigue strength under reversed bending stresses is achieved.
The invention further relates to a device for the heat treatment of profiled rolling stock, in particular track or railroad rails, essentially including at least one stand-by area for the rolling stock at the roller table, with a rolling stock positioning device, a cooling treatment area, with devices for partial high intensity heat removal from the surface of the rolling stock and a final cooling area for cooling the rolling stock to room temperature, as well as depositing, transverse transporting, stopping and manipulating device.
Finally, the invention relates to profiled rolling stock, in particular to a track or railroad rail, including of a rail head of an at least partial pearlitic grain structure, a rail base and a web between the rail head and the rail base.
2. Discussion of Background Information
Profiled rolling stock, in particular track or railroad rails, is mainly produced from basic iron alloys with weight-% contents between 0.4 and 1.0 C, 0.1 and 1.2 Si, 0.5 and 3.5 Mn, if required up to 1.5 Cr, as well as other alloy elements at concentrations below 1%, the rest being iron and impurities occurring in the manufacturing process. Based on the usual dimensions, for example a weight between 30 to 100 kg/m, and the ratio of cross section to circumference of rails resulting therefrom, during cooling of the rolling stock from the conversion heat in still air, for example on cooling beds and the like, a conversion of the grain from an austenitic into a rough pearlitic structure, possibly having portions of ferrite, because of slow cooling, takes place. The previously mentioned materials having the above structure have a hardness in the range between 250 HB to 350 HB.
An increase in traffic and larger axial loads, as well as the desire to improve the durability of rails in practical use has resulted in a multitude of suggestions for increasing the strength and wear resistance of the material. In the course of this it is possible to achieve more advantageous or improved material properties with a hardness of 400 HB and above by measures in respect to heat treatment and/or alloy techniques.
However, rails should be easy to weld in the field for reasons, among others, of forming shock-free sections or multiple lengths, so that measures in respect to alloy techniques for increasing the hardness or strength and durability of the material can mostly be applied on a small scale only due to the welding problems and are aimed to a heat treatment matched to the composition of the steel (German Patent Publication DE-C 34 46 794, European Patent Publications EP-B-0 187 904, EP-B-0 186 373). For economic reasons, such methods have also not proven themselves on a large scale.
To increase the useful properties of rails and switch parts made from the above mentioned materials it is possible and known to one skilled in the art to provide a fine pearlitic material structure by a thermal tempering treatment. In the process it is important to set appropriate cooling conditions or cooling rates for the cool-down from the austenitizing temperature. For example, European Patent Publication EP-B-0 293 002 suggests for this purpose to perform, after an initially high cooling intensity, a practically isothermic structural conversion at approximately 530.degree. C. It is furthermore known from German Published, Non-Examined Patent Application DE-OS 28 20 784 to perform hardening of rails of a defined composition in boiling water and to achieve a desired cooling intensity for setting a fine pearlitic structural state by additives and movement steps.
In accordance with Austrian Patent AT-PS-323 224 it had also been suggested to produce rails with a homogeneous fine pearlitic structure from a selected alloy by the application of defined cooling parameters, for example a cooling speed between 10 and 20.degree. C./s down to a temperature of no more than 550.degree. C. However, the above steps have the common disadvantage that, depending on the mass concentration of the rolling stock profile, an even cooling intensity of the surface can cause different cooling speeds and structural forms in the zones close to the surface, and that it is often necessary to take elaborate precautions to prevent undesired local structural form or material properties, in particular excessive hardness and brittleness, in parts of the rail which are primarily stressed by bending.
In many cases it was also proposed to provide in a directed manner a heterogeneous microstructure in the cross section of a rail in accordance with the respective stresses. For example, a method is known from German Patent Publication DE-C-30 06 695, in accordance with which a conversion over the entire cross section is caused from the rolling heat by cooling the rail, after which the head of the rail is re-austenitized by inductive heating and subsequently hardened. In accordance with WO 94/02652 it was further proposed to cool the rail head to a surface temperature between 450 and 550.degree. C. in a cooling medium of a specially set cooling intensity and in this way to create a fine pearlitic grain therein. A device for the suspended hardening of rails in accordance with German Patent Publication DE-C-40 03 363 is suitable for such treatment.
However, the inhomogeneous cooling over the cross section of profiled rolling stock can lead to curvatures or deviations from the straightness at room temperature. To avoid this disadvantage it has been proposed (German Patent Publication DE-A-42 37 991) to transport or cool rails suspended, preferably with the head down, on a cooling bed, however, a directed formation of a heterogeneous grain structure over the cross section is hardly possible here.
All of the methods and devices known up to now have the common disadvantage that although they disclose solutions in limited areas or regarding individual method steps leading to the desired goal in the manufacture of profiled rolling stock, overcoming all the problems in a satisfactory way cannot be shown in connection with an economical production of long rails of high quality and with special finishing properties.